When a new prisoner enters jail, the other cons will instantly become aware that he is different- he is usually overweight, middle aged, balding and a social recluse. Because he is not the usual 'hard case' prisoner, the others will become aware that something is wrong. After a brief investigation, the prisoners will, correctly or incorrectly, assume he is a nonce (one who fiddles kids). This will rapidly lead to a 'nonce bashing' - an extremely violent form of vigilante justice, performed away from the guards, in some dark corner of the jail. This usually leads to death or serious brain damage.
Only performed on sex offenders, never on those who may have committed 'worse' crimes such as murder or regicide, because these are more 'manly' and commmon crimes. also a murderer is likely to put up more of a fight than a nonce.
CONVICT2- "He's a nonce."
CONVICT3- "Fuckin' 'ell, LET'S BASH THE NONCE BASTARD!"